1. Field of the Invention
The present invention relates to a projection lens, and, more particularly, to a projection lens suitable for use in, for example, a projector of a projection display device.
2. Description of the Related Art
In recent years, the use of a projection display device is becoming widespread.
What is called a rear projection display device is known as one type of projection display device. In the rear projection display device, image light is projected onto a transmissive screen from the back surface side thereof in order to display an image.
What is called a three-plate projection display device which includes two-dimensional image display devices (that is, light valves) provided in correspondence with three colors, red (R), green (G), and blue (B), is known as one type of rear projection display device. In the three-plate projection display device, light beams from a white light source and collimated by, for example, a reflector is separated into light beams of the three different colors, a red light beam, a green light beam, and a blue light beam, by a color-separating mirror.
Then, the light beams of the three different colors impinge upon the corresponding two-dimensional image display devices (such as liquid crystal displays (LCD)) formed in correspondence with a red (R) image electrical signal, a green (G) image electrical signal, and a blue (B) image electrical signal. Image light beams obtained at the two-dimensional image display devices formed in correspondence with their respective colors, red, green, and blue, are synthesized into a white image light beam by a color synthesizing optical system in order to be projected in enlarged form onto a transmissive screen through a projection lens.
Another type of three-plate projection display device is known. In this type, light beams emitted from light sources (such as light-emitting diodes (LED) or lasers) which emit light beams of three different colors, a red light beam, a green light beam, and a blue light beam, are collimated. The collimated light beams are made to impinge upon corresponding two-dimensional image display devices formed in correspondence with a red (R) image electrical signal, a green (G) image electrical signal, and a blue (B) image electrical signal. Image light beams obtained at their corresponding two-dimensional image display devices formed in correspondence with their respective colors, red (R), green (G), and blue (B), are synthesized into a white image light beam by a color synthesizing optical system in order to be projected in enlarged form onto a transmissive screen through a projection lens.
What is called a single-plate projection display device which includes one two-dimensional image display device is also known.
In one type of known single-plate projection display device, light sources which emit light beams of three different colors, a red light beam, a green light beam, and a blue light beam, are made to emit or transmit light based on time division. Then, in accordance with the timings of the passages of the light beams of the three different colors from the light sources, driving operations using a red (R) image electrical signal, a green (G) image electrical signal, and a blue (B) image electrical signal corresponding thereto are performed on one two-dimensional image display device in order to project an image in enlarged form onto a transmissive screen through a projection lens.
In another type of known single-plate projection display device, color filters for passing light beams of three different colors, red, green, and blue, are formed with every one pixel on a panel. A white light beam is made to impinge upon one two-dimensional image display device. Then, the light which has passed through the two-dimensional image display device is projected in enlarged form onto a transmissive screen through a projection lens.
In still another type of known single-plate projection display device, white light is made to impinge upon three dichroic mirrors disposed at very small angles. The light beams are separated into three different color light beams with each angle in order to obtain a red light beam, a green light beam, and a blue light beam. The obtained light beams are made to impinge upon one two-dimensional image display device. On the two-dimensional image display device, the light beams are collimated by very small lenses formed in correspondence with sets of three image pixels formed in correspondence with the three colors, red (R), green (G), and blue (B). Each color pixel is driven by its corresponding color image electrical signal. The light beams which have impinged upon their corresponding pixels are projected in enlarged form onto a transmissive screen through a projection lens.
Considering, for example, the limitations caused by a quick return mirror, various lenses having structures similar to that of the projection lens installed in each of the above-described projection display devices have been proposed. They include a wide-angle photographic lens for a single-lens reflex camera having a long back focus and a wide-angle projection lens for a projection television using a CRT (cathode ray tube).
In the structure of each projection display device described above, an optical element, such as a dichroic prism or a dichroic mirror, may be disposed as a color-light synthesizing system. In the case where a reflective, two-dimensional image display device is used, an optical element, such as a polarization beam splitter prism or a polarization beam splitter mirror, may be disposed. In these cases, the back focus which corresponds to the distance from the two-dimensional image display device to the last end of the projection lens needs to be made long.
In a projection display device which forms an image in enlarged form on the entire transmissive screen using one projector, in order to make the projection display device itself more compact, it is necessary to reduce the projection distance (for example, the length of the center light beam measured from a light-exiting end of the projection lens to the transmissive screen through a projection lens). To reduce the projection distance, it is necessary to increase the divergence angle of the exiting lens as a result of forming the projection lens into a wide-angle lens in order to form a large screen.
In order to improve color uniformity on the screen onto which image light beams are projected, it is preferable that the angle width of the light beams which strike the coated surfaces of, for example, a polarization beam splitter prism or a polarization beam splitter mirror (used when a dichroic prism or a dichroic mirror, used as a color synthesizing optical system, and a reflective two-dimensional image device are used) be fixed.
Therefore, the projection lens needs to be telecentric so that a principal ray from abaxial point to the projection lens is perpendicular to the two-dimensional display device. Here, the projection lens is symmetrical with respect to the optical axis which passes through the center of the two-dimensional display device, whereas the two-dimensional display device itself has a high contrast in only one direction. Therefore, the light beam itself which is shed upon the two-dimensional display device needs to be shed at an angle.
For a two-dimensional image display device, a liquid crystal display (LCD) or the like is ordinarily used. The liquid crystal display is driven using a matrix electrode. Therefore, unlike the case where a cathode ray tube (CRT) is used, it is difficult to correct distortion of the projection lens. More specifically, when a cathode ray tube is used, distortion of the projection lens can be relatively easily corrected by using a function for correcting a raster form such as a pincushion distortion correction function. However, when a liquid crystal display device which performs a dot-matrix display operation is used, corrections of raster distortion are ordinarily not carried out.
From the above, it is preferable that distortion of the projection lens be as small as possible. However, this is a barrier to obtaining a wide-angle lens having a long back focus.
In other words, when the projection lens which is formed so as to have a wide angle and a long back focus is made telecentric, the overall length and the diameter of the lens tend to become large.
In a wide-angle photographic lens for a single-lens reflex camera and a projection lens for a projection television using a cathode ray tube, the back focal length is insufficient and the incident angle and the exiting angle of an abaxial light beam are large, so that, in the present condition, they are not telecentric and the relative illuminance at corner is small.
In recent years, there has been a demand for a high-resolution lens in correspondence with the production of finer light valves. The production of lenses having higher resolution has caused the problem of color misalignment in the pixels due to chromatic aberration at a peripheral portion of the screen.
In a projection display device having a structure in which an optical path within a projection lens is changed, what is called a total drawing-out method is used as a focusing method. In this method, the position of the focus is obtained by adjusting, for example, the distance between the whole projection lens and the screen relative to each other. When this method is used, the center of the image on the screen is shifted, so that it is known that the total drawing-out method is not an appropriate method. Therefore, in such a projection display device, a focusing method other than the total drawing-out method needs to be used.
In a projection display device, even if a housing having a size which is different from the size of the screen is used, the same projection lens can be used by adjusting the distance between the projection lens and the screen relative to each other. Here, aberrations (such as distortion and chromatic aberration of magnification) occur as a result of, for example, slight differences in the angles of the light beams which converge on the screen or errors in producing projection lenses. Therefore, it is necessary to make adjustments so that aberrations are made as small as possible.
From the point of view of costs, it is preferable to cut costs of the projection lens itself. One method of achieving this is to reduce the number of lenses making up the projection lens.
Accordingly, in view of the above-described problems, it is an object of the present invention to provide a projection lens which provides high optical performance to the extent possible even if few lenses are used. In other words, it is an object of the present invention to provide a wide-angle projection lens which is telecentric and has a long back focus even if the projection distance is short, and whose aberrations are small.
To this end, according to a first aspect of the present invention, there is provided a projection lens comprising a first lens group, a diaphragm, and a second lens group, which are disposed in that order from a long conjugate side to a short conjugate side. The first lens group comprises an aspherical lens disposed at a long conjugate side, and at least one positive lens, so that the first lens group as a whole has a positive refractive power. The second lens group comprises at least one set of combined lenses, and an aspherical lens, so that the second lens group as a whole has a positive refractive power. When the first lens group and the second lens group have these structures and properties, a focusing operation is carried out by moving a gap between predetermined lenses in the first lens group, and the following conditional expressions are satisfied:
1.87 less than BF/F
xe2x80x830.22 less than HF2/F2 less than 0.57
where F denotes the focal length of the whole system, BF denotes the distance in air from a last lens surface of the second lens group to a small conjugate point when the projection magnification is a predetermined value, F2 denotes the focal length of the second lens group, and HF2 denotes the location of a front principal point of the second lens group.
According to a second aspect of the present invention, there is provided a projection lens comprising a first lens group, a diaphragm, and a second lens group, which are disposed in that order from a long conjugate side to a short conjugate side. The first lens group comprises a convex-shaped meniscus lens disposed at the long conjugate side, and at least one positive lens, so that the first lens group as a whole has a positive refractive power. The second lens group comprises at least one set of combined lenses and an aspherical lens, so that the second lens group as a whole has a positive refractive power. When the first lens group and the second lens group have these structures and properties, a gap between predetermined lenses of the first lens group is moved in order to perform a focusing operation, and the following conditional expressions are satisfied:
1.87 less than BF/F
0.22 less than HF2/F2 less than 0.57
where F denotes the focal length of the whole system, BF denotes the distance in air from a last lens surface of the second lens group to a small conjugate point when the projection magnification is a predetermined value, F2 denotes the focal length of the second lens group, and HF2 denotes the location of a front principal point of the second lens group.
According to a third aspect of the present invention, there is provided a projection lens comprising a first lens group, a diaphragm, and a second lens group, which are disposed in that order from a long conjugate side to a short conjugate side. The first lens group comprises two lens subgroups comprising two lenses, an aspherical lens disposed at a long conjugate side, and a positive lens, so that the first lens group as a whole has a positive refractive power. The second lens group comprises two lens subgroups comprising three lenses, a set of combined lenses and an aspherical lens, so that the second lens group as a whole has a positive refractive power. When the first lens group and the second lens group have these structures and properties, a gap between predetermined lenses in the first lens group is moved in order to perform a focusing operation, and the following conditions are satisfied:
1.87 less than BF/F
0.22 less than HF2/F2 less than 0.57
where F denotes the focal length of the whole system, BF denotes the distance in air from a last lens surface of the second lens group to a small conjugate point when the projection magnification is a predetermined value, F2 denotes the focal length of the second lens group, and HF2 denotes the location of a front principal point of the second lens group.
According to a fourth aspect of the present invention, there is provided a projection lens comprising a first lens group, a diaphragm, and a second lens group, which are disposed in that order from a long conjugate side to a short conjugate side. The first lens group comprises two lens subgroups comprising two lenses, an aspherical lens disposed at a long conjugate side, and a positive lens, so that the first lens group as a whole has a positive refractive power. The second lens group comprises either two lens subgroups or three lens subgroups, the two lens subgroups comprising three lenses, an aspherical lens disposed closest to the diaphragm, and combined lenses, and the three lens subgroups comprising four lenses, an aspherical lens disposed closest to the diaphragm,a positive lens, and combined lenses, so that the second lens group as a whole has a positive refractive power. When the first lens group and the second lens group have these structures and properties, a gap between predetermined lenses in the first lens group is moved to perform a focusing operation, and the following conditions are satisfied:
1.87 less than BF/F
0.22 less than HF2/F2 less than 0.57
where F denotes the focal distance of the whole system, BF denotes the distance in air from a last lens surface of the second lens group to a small conjugate point when the projection magnification is a predetermined value, F2 is the focal length of the second lens group, and HF2 is the location of a front focal point of the second lens group.
According to a fifth aspect of the present invention, there is provided a projection lens comprising a first lens group, a diaphragm, and a second lens group, which are disposed in that order from a long conjugate side to a short conjugate side. The first lens group comprises either two lens subgroups or three lens subgroups, the two lens subgroups comprising two lenses, an aspherical lens disposed at a long conjugate side, and a positive lens, and the three lens subgroups comprising three lenses, an aspherical lens disposed at the long conjugate side, a negative meniscus lens, and a positive lens, so that the first lens group as a whole has a positive refractive power. The second lens group comprises two lens subgroups including three lenses, a set of combined lenses and an aspherical lens disposed at a short conjugate side, so that the second lens group has as a whole a positive refractive power. When the first lens group and the second lens group have these structures and properties, a gap between predetermined lenses in the first lens group is moved to perform a focusing operation, and the following conditional expressions are satisfied:
1.87 less than BF/F
0.22HF2/F2 less than 0.57
where F is the focal length of the whole system, BF is the distance in air from a last lens surface of the second lens group to a small conjugate point when the projection magnification is a predetermined value, F2 is the focal length of the second lens group, and HF2 is the location of a front principal point of the second lens group.
According to the present invention, when the lenses in each of the above-described structures are arranged as described above, and satisfy each of the conditional expressions, the conditions for obtaining a projection lens which has its projection distance kept short while having a large angle of view and a long back focus, and which has its telecentric property maintained when few lenses are used are satisfied.